1. Field of the Invention
The present invention relates generally to underground locators, and more specifically to portable above-ground equipment that can indicate the depth and lateral position of selected underground pipes and cables radio-illuminated by a tag transmitter or other source.
2. Description of the Prior Art
The public safety and convenience depend on contractors being able to accurately understand where underground utilities are buried and how deep they are placed. Otherwise, gas, water, and sewer pipes can be punctured, or telephone, electric, or CATV cables can be cut when an excavator accidentally comes across the service. Some such accidents have resulted in wide-area outages, and even direct injury and death to nearby construction workers, passersby and residents. Many jurisdictions therefore require contractors and other excavators to contact a "one-call" service before digging so that a crew can come out and mark the existing underground structures to be avoided. Unfortunately, the prior art locating equipment has often not been up to the job of providing sufficiently accurate and reliable depth measurements, and so these jurisdictions usually stop short of having the underground depths marked.
A prior art method of measuring the depth of an underground pipe or cable radio-illuminates the hidden structure by attaching a transmitter to some part of it. For example see, U.S. Pat. No. 3,991,363, issued to Thomas Lathrop on Nov. 9, 1976. A portable receiver with at least two vertically separated antennas is walked above ground at the surface until the transmitter's signal is intercepted. The portable receiver is then repositioned directly over the radio-illuminated underground pipe or cable. Since the signal strength of the electromagnetic radiations can be depended upon to attenuate with the distance traveled, a pair of field strength measurements taken at known elevations can be used to compute the depth of the source radiator. For an infinite line radiator, the signal strength will diminish as the inverse of the distance.
An early prior art attempt to make the depth of underground conductors continuously available from a locator is described by Mark Howell, et al., in U.S. Pat. No. 4,220,913, issued Sep. 2, 1980. The ratio of the signal amplitudes received by a pair of horizontal coil antennas is used for a depth indicator. But such a simple mechanism was plagued by amplifiers that were hard to match and that drifted, and a variety of noise sources some of which were more than enough to obscure the signal so much that the underground-conductor depth indication was not very reliable.
The present inventor, Earl Peterman, described one way to address the mismatch of channel amplifiers for the top and bottom signal sensors in U.S. Pat. No. 4,520,317, issued May 28, 1985. A shared automatic gain-controlled amplifier is switched between the top and bottom antennas and the difference in readings is assumed to be the result of the two different field strengths experienced by each sensor. Any channel amplifier offsets and gain errors will be the same for each reading because the same channel amplifier was common to both readings. A practical limitation that was encountered with this circuit design was its continued susceptibility to noise. The field strengths of the signals of interest can be on the order of a few microvolts per meter, and measuring the small differences in field strength associated with the deeper objects has proved to be very challenging.
A method for the determination of the difference in field strengths of the signals received by the top and bottom sensors was described by present inventors, Earl and David Peterman, in U.S. Pat. No. 5,001,430, issued Mar. 19, 1991. The common gain-controlled amplifier switched between the top and bottom antennas was described as being digitally controlled. A reference level is established at one gain setting for the bottom antenna, and then that same level is matched for a reading from the top antenna by digitally adjusting the gain settings. The difference in field strengths for the top and bottom antennas is obtained by the digital difference of the gain settings needed to make the received signals equal.
It has now universally developed in most commercial depth-finding locators in the United States that the operators must stop and press a button in order to obtain a depth measurement. This mode of operation has resulted from having to share a single amplifier between the top and bottom sensor channels. But a continuous mode of depth measurement is needed in order to improve operator efficiency and increase confidence that a particular pipe or cable is being tracked from above at the ground surface.